A dipole antenna called a "sleeve antenna" has been used as an antenna for a base station in mobile radio. In FIG. 15, an example of a sleeve antenna in the prior art is illustrated (see, for example, Laid-open Japanese Patent Application No. (Tokkai hei) 8-139521). As shown in FIG. 15, outside an outer conductor 50a of a coaxial feed line 50, a 1/4-wavelength sleeve-like metal pipe 51 is located with one end connected to the upper end of outer conductor 50a. Also, an inner conductor 50b of coaxial feed line 50 protrudes from the upper end of outer conductor 50a, and a 1/4-wavelength antenna element 52 is connected to the protruding inner conductor 50b. Thus, a 1/2-wavelength dipole antenna 53 is formed. Also, another example of a sleeve antenna is disclosed in Laid-open Japanese Patent Application No. (Tokkai hei) 4-329097, and it has a structure as shown in FIG. 16. In FIG. 16, a dipole antenna 57 comprises an antenna element 55 formed by extending an inner conductor 55 of a coaxial feed line 54 upward by a length corresponding to about a 1/4 wavelength from the upper end of an outer conductor, and a 1/4-wavelength sleeve-like metal pipe 56 located outside coaxial feed line 54 with one end connected to the upper end of the outer conductor. A passive element 59 is supported by a supporting means mounted to metal pipe 56.
Also, a "colinear array antenna", a vertically polarized plane wave omnidirectional antenna having a large gain, has been used as an antenna for a base station in mobile radio. A colinear array antenna in the prior art is disclosed in Laid-open Japanese Utility Model Application No. (Tokkai hei) 2-147916, and has a structure as shown in FIG. 17. In FIG. 17, in an outer conductor 60a of a coaxial feed line 60, an annular slit 61 is provided at predetermined spacing. Outside outer conductor 60a of coaxial feed line 60, a pair of 1/4-wavelength sleeve-like metal pipes 62 is located on both sides of each annular slit 61. Thus, a plurality of dipole antenna elements 63 are formed. Between the lowest dipole antenna element 63 and an input terminal 64, a plural-stage 1/4-wavelength impedance conversion circuit 65 is provided for impedance matching. Also, in FIG. 17, 60b denotes an inner conductor of coaxial feed line 60.
In the sleeve antenna as shown in FIG. 15, the coaxial feed line does not affect the antenna characteristics when the antenna is used as a vertically polarized plane wave antenna. However, the sleeve-like metal pipe forms a balun, and therefore the antenna is a narrow band antenna. Thus, the antenna must be adjusted to have a band that is sufficiently broader than a desired band in view of a difference in the resonance frequency of the antenna that may result due to a variation in the size of a component and a variation in finished size in the manufacturing process. In this case, making the diameter of a sleeve-like metal pipe large is one way to implement a broad band. However, if the diameter of the sleeve-like metal pipe is large, the antenna becomes heavier, and therefore supporting metal fittings provided in a base station become large.
In the sleeve antenna as shown in FIG. 16, a directional pattern can be set in any direction by the passive element. Therefore, the antenna is an antenna for a base station that is effective in covering only the range of a specific direction in indoor location, for example. However, in the above structure, the dipole antenna and the passive element are exposed, and therefore the structure is not sufficient for weather resistance and mechanical strength in outdoor location. Furthermore, this structure requires a supporting means for the passive element, and therefore the manufacturing is troublesome.
Generally, in a colinear array antenna having a large gain that is used in a base station, a standing wave ratio (SWR) in a used frequency band is required to be 1.5 or less. In order to implement this, a plural-stage 1/4-wavelength impedance conversion circuit is provided to perform impedance matching in the conventional structure as mentioned above (FIG. 17). Therefore, the structure is complicated, and the entire length of the antenna is long. These problems are factors that prevent the small size and low cost for a base station, while base stations are increasingly installed for securing the number of channels for mobile radio.